Multilayer oxygen barrier packaging film

ABSTRACT

A thermoplastic multilayer packaging film has a core layer including ethylene/vinyl alcohol copolymer; two outer layers, and two adhesive layers each disposed between the core layer and a respective outer layer, characterized in that at least one of the outer layers includes a blend of A, B, and C, wherein component A is a homogeneous or heterogeneous ethylene/alpha olefin copolymer with density comprised greater than 0.915 g/cm 3  and less than 0.925 g/cm 3 , component B is a homogeneous or heterogeneous ethylene/alpha olefin copolymer with density greater than or equal to 0.925 g/cm 3 , and component C is a homogeneous or heterogeneous ethylene/alpha olefin copolymer with density less than or equal to 0.915 g/cm 3 . The use of modified homogeneous ethylene/alpha olefin copolymers with a density of from 0.900 g/cm 3  to 0.908 g/cm 3  as adhesive layers to improve the interlayer adhesion and the sealability properties of films with a core layer comprising an ethylene/vinyl alcohol copolymer or a polyamide and outer layers comprising an ethylene/alpha olefin copolymer is also described.

FIELD OF THE INVENTION

The present invention refers to a multilayer film endowed with goodoptical, mechanical, gas barrier, and heat sealability properties. Thefilm is particularly suitable for packaging food products.

BACKGROUND OF THE INVENTION

Multilayer, thermoplastic films are being used for packaging variousfood and non food products because they protect the item itself from theenvironment during storage and distribution. Furthermore, for the endconsumer, it is desirable to present the product packaged in apreferably transparent thermoplastic film that allows visible inspectionof the package contents to help assure the quality of the product.

Optical characteristics are therefore often important for athermoplastic film for packaging.

Other properties are also desirable, such as good mechanical propertiesthat keep the package unaltered until it is offered to the customer onconsumer.

A shrink feature can also be imparted to a thermoplastic film byorientation or stretching of the film, either mono-axially or biaxially,during film manufacture. This shrink feature allows the film to shrinkor, if restrained, create shrink tension within the film upon exposureto heat. In a typical process, the thick structure which is extrudedthrough either a round or a flat extrusion die is quickly quenched, thenit is heated to a suitable temperature, called the orientationtemperature, which is higher than the glass transition temperature(T_(g)) of the resins used in the film itself but lower than the meltingtemperature (T_(m)) of at least one of the resins, and stretched ineither or both of the machine (longitudinal) and transverse directions.

For food packaging, it is often necessary that the film has oxygenbarrier characteristics to delay or avoid product oxidation ordegradation during its shelf-life.

Good heat sealability is sometimes required. It is essential,particularly for oxygen barrier films used in applications where thecontained product is to be kept either under vacuum or under a modifiedatmosphere, that the seal(s) that close the package have adequatestrength and, as a consequence thereof, that the package remains tight.

Several different materials have been used to decrease the oxygenpermeability of thermoplastic films. Among these materials a very goodgas barrier material is EVOH (ethylene/vinyl alcohol copolymer). Several“barrier” thermoplastic films comprising an EVOH layer are described inthe patent literature.

In prior art films the different materials employed for the skin layersare suitably combined with the aim of improving as much as possible thefilm characteristics, particularly those characteristics that are neededfor the specific intended applications. As an example, the use of a lowdensity ethylene/alpha olefin copolymer provides for fairheat-sealability and remarkable oil resistance properties; the use ofEVA (ethylene/vinyl acetate copolymer) improves the shrinkability andthe sealability properties; the use of propylene homo- and/or copolymerincreases the stiffness of the structure; etc.

It is however known that a resin that improves a specific propertytypically worsens other properties, and therefore the research effortsin this field tend to reach an optimum balance of these properties.

More particularly the film characteristics that still need to beimproved, in a way that however should not negatively affect the othercharacteristics such as optical, mechanical, barrier and shrinkabilityproperties, are the sealability properties.

It has now been found that it is possible to provide a film with opticaland gas barrier properties at least comparable to those of known filmscontaining EVOH, and having remarkably improved mechanical andheat-sealability properties, by using in the film sealing layer a blendof ethylene/alpha olefin copolymers of suitably selected differentdensities.

SUMMARY OF THE INVENTION

In one aspect, a multilayer thermoplastic film comprises a core layercomprising an ethylene/vinyl alcohol copolymer; two outer layers; andtwo adhesive layers each disposed between the core layer and arespective outer layer; wherein at least one of the outer layerscomprises a blend of a homogeneous or heterogeneous ethylene/alphaolefin copolymer having a density of greater than 0.915 g/cm³ and lessthan 0.925 g/cm³, a homogeneous or heterogeneous ethylene/alpha olefincopolymer having a density greater than or equal to 0.925 g/cm³, and ahomogeneous or heterogeneous ethylene/alpha olefin copolymer having adensity less than or equal to 0.915 g/cm³.

Only one of the outer layers (the outer layer that will be used as thesealing layer) needs to comprise a blend as defined above. The otherouter layer can have a different composition and comprise a singlepolymer or a blend of a polymers typically selected from ethylene homo-and co-polymers, e.g. polyethylene, homogeneous or heterogeneousethylene/alpha olefin copolymers, EVA, etc. In a preferred embodiment,however, both outer layers comprise a blend as defined above.

Films of the invention comprise at least five layers. Films with ahigher number of layers, either symmetrical or unsymmetrical, areobtained when one or more additional layers are present between theadhesive layers and one or both of the outer layers, and/or between thecore layer and one or both of the adhesive layers.

Preferably, the adhesive layers directly adhere to the core layer.

When, in said preferred embodiment, the surface of each of the adhesivelayers that does not adhere to the core layer, directly adheres to therespective outer layer, the film will contain five layers.

Films can also contain a higher number of layers if one or moreadditional layers are positioned between the adhesive layers and theouter layers. As an example, a six or seven layer film can compriseadditional layer(s) between the adhesive layers and the outer layers,the additional layers made with recycle material from the scrap of thesame film, optionally blended with a compatibilizer.

In a second aspect, a multilayer thermoplastic film comprises a corelayer comprising an ethylene/vinyl alcohol copolymer, the core layerhaving two major surfaces; two intermediate layers each comprisingpolyamide, each of which is directly adhered to one of the two majorsurfaces of the core layer; two outer layers; and two adhesive layerseach disposed between a respective intermediate layer and a respectiveouter layer; wherein at least one of the outer layers comprises a blendof a homogeneous or heterogeneous ethylene/alpha olefin copolymer havinga density of greater than 0.915 g/cm³ and less than 0.925 g/cm³, ahomogeneous or heterogeneous ethylene/alpha olefin copolymer having adensity greater than or equal to 0.925 g/cm³, and a homogeneous orheterogeneous ethylene/alpha olefin copolymer having a density less thanor equal to 0.915 g/cm³.

The film of this embodiment is thus a seven layer film, when each of theadhesive layers is directly adhered to a respective intermediate layeras well as to a respective outer layer. It may also comprise more thanseven layers when additional layers, such as layers made with recyclematerial, are present between the adhesive layers and the outer layers.

DEFINITIONS

As used herein, the term:

“film” refers to a flat or tubular flexible structure of thermoplasticmaterial having a thickness up to about 120 μm. Generally, for thepurposes of the present invention, said structure will have a thicknessof up to about 60 μm and typically up to about 35 μm;

“core layer” or “inner layer” refer to any film layer having its twoprincipal surfaces adhered to other layers of the multilayer film;

“outer layer” or “skin layer” refers to any film layer of a multilayerfilm having only one of its principal surfaces directly adhered toanother layer of the film;

“heat-sealing” or “heat-sealant” layer, as applied to multilayer films,refers to an outer layer which is involved in the sealing of the film toitself, to another film layer of the same or another film, and/or toanother article which is not a film;

“adhesive layer” or “tie layer” refer to any inner layer having theprimary purpose of adhering two layers to one another;

“directly adhered” as applied to film layers is defined as adhesion ofthe subject film layer to the object film layer, without a tie layer, anadhesive or other layer in between. In contrast, as used herein, theword “between”, as applied to a film layer expressed as being betweentwo other specified layers, includes both direct adherence of thesubject layer to the two other layers it is between, as well as a lackof direct adherence to either or both of the two other layers thesubject layer is between, i.e. one or more additional layers can bepresent between the subject layer and one or more of the layers thesubject layer is between;

“heat-shrinkable” film refers to a film drawn mono-axially or biaxiallythat upon heating for 5 seconds at a temperature of 120° C. shows a freeshrink of at least 10% in at least one direction;

“homopolymer” refers to a polymer resulting from the polymerization of asingle monomer, i.e. a polymer consisting essentially of a single typeof repeating unit;

“copolymer” refers to a product of a polymerization reaction involvingtwo or more different comonomers;

“polyolefin” refers to a thermoplastic resin obtained by polymerizationof an olefin, or by copolymerization of two or more olefins or of one ormore olefins with other comonomers, wherein the olefin units are presentin larger amounts than any possibly present comonomer. Suitable examplesof “polyolefins” are polyethylene, ethylene/alpha olefin copolymer(either heterogeneous or homogeneous), ethylene/vinyl acetate copolymer,ethylene/acrylic acid or methacrylic acid copolymers, etc.;

“modified polyolefin” refers to a polyolefin characterised by thepresence of functional groups such as anhydride or carboxy groups.Examples of said modified polyolefins are graft copolymers of maleicacid or maleic acid anhydride onto ethylene/alpha olefin orethylene/vinyl acetate copolymer, polymerisation products of these withother polar monomers, blends thereof, etc.;

“EVOH” or the phrase “ethylene/vinyl alcohol copolymer” refer tosaponified or hydrolysed products of ethylene/vinyl ester copolymer,generally of ethylene/vinyl acetate copolymer, wherein the ethylenecontent is typically between 20 and 60 mole %, preferably between 28 and49 mole %, and the degree of saponification is higher than 90%,preferably higher than 95%;

“polyamide” refers to high molecular weight polymers having amidelinkages along the molecular chain. Such term encompasses bothpolyamides and copolyamides with aliphatic and/or aromatic repeatingunits, either crystalline, semi-crystalline or amorphous;

“ethylene/alpha olefin copolymer” refers to a copolymerization productof ethylene with one or more alpha olefins, e.g. butene-1, hexene-1,methyl-4-pentene-1, octene-1, as well as blends thereof. Said phraseincludes both heterogeneous and homogeneous ethylene/alpha olefincopolymers;

“heterogeneous ethylene/alpha olefin copolymer” refers to thosepolymerization reaction products characterised by a relatively widevariation in molecular weight and composition distribution. Suchheterogeneous polymers typically contain a relatively wide variety ofchain lengths and comonomer percentages. The molecular distribution,expressed as M_(w)/M_(n) wherein M_(w) is the weight average molecularweight, and M_(n) is the number average molecular weight, is higher than3. These heterogeneous polymers are typically prepared by using theconventional Ziegler-Natta catalysts in heterogeneous phase. Dependingon the density these copolymers are generally indicated by theabbreviations LMDPE (linear medium density polyethylene—thatconventionally designates heterogeneous ethylene/alpha olefin copolymershaving a density greater than or equal to 0.925 g/cm³), LLDPE (linearlow density polyethylene—that conventionally designates heterogeneousethylene/alpha olefin copolymers having a density of from 0.915 g/cm³ to0.925 g/cm³), and VLDPE (very low density polyethylene—thatconventionally designates heterogeneous ethylene/alpha olefin copolymershaving a density less than or equal to 0.915 g/cm³);

“homogeneous ethylene/alpha olefin copolymers” refers to polymerizationreaction products of relatively narrow molecular weight distribution andrelatively narrow composition distribution. Such homogeneous polymersstructurally differ from heterogeneous polymers in that they exhibit arelatively even sequencing of comonomers within a chain, the mirroringof sequence distribution in all chains, and the similarity of length ofall chains. With a few exceptions (such as the homogeneous linearethylene/alpha olefin copolymers named TAFMER™ that are manufactured byMitsui Petrochemical Corporation using homogeneous Ziegler-Nattacatalysts), homogeneous polymers are generally prepared using“metallocene”, or “single-site”, or “constrained-geometry” catalysts.Homogeneous polymers can be identified and classified by molecularweight distribution (M_(w)/M_(n)), and composition distribution breathindex (CDBI). Molecular weight distribution, also known aspolydispersity, may be determined by gel permeation chromatography.Homogeneous ethylene/alpha olefin copolymers useful in the presentinvention have a (M_(w)/M_(n)) of less than about 3. The CDBI of suchhomogeneous ethylene/alpha olefin copolymers will be greater than 60%,e.g. greater than 70%. CDBI is defined as the weight percent of thepolymer molecules having a comonomer content within 50 percent (i.e.plus or minus 50%) of the median total molar comonomer content. The CDBIof a polyethylene homopolymer, which does not contain a comonomer, is bydefinition 100%. The CDBI is readily calculated from data obtained fromtechniques known in the art, such as Temperature Rising ElutionFractionation (TREF) as described for instance by Wild et al. in Journalof Polymer Science, Poly. Phys. Ed., Vol. 20, p.441 (1982). Homogeneousethylene/alpha olefin copolymers, obtained by using metallocenecatalysts, are commercialised by Exxon Chemical Company under the tradename EXACT™, by BASF as LUFLEXEN™, and by Dow as AFFINITY™ or ENGAGE™resins.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be further understood with reference to the drawingswherein FIGS. 1 and 2 are schematic cross-sections of variousembodiments of a film of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, film 10 comprises a core layer 12 comprisingan ethylene/vinyl alcohol copolymer, two outer layers 18 and 20, and twoadhesive layers 14 and 16 each disposed between the core layer 12 and arespective outer layer, wherein at least one of the outer layers 18 and20 comprises a blend of a homogeneous or heterogeneous ethylene/alphaolefin copolymer having a density of greater than 0.915 g/cm³ and lessthan 0.925 g/cm³, a homogeneous or heterogeneous ethylene/alpha olefincopolymer having a density greater than or equal to 0.925 g/cm³, and ahomogeneous or heterogeneous ethylene/alpha olefin copolymer having adensity less than or equal to 0.915 g/cm³.

With reference to FIG. 2, film 30 comprises a core layer 12 comprisingan ethylene/alpha alcohol copolymer, the core layer having two majorsurfaces; two intermediate layers 42 and 44 each comprising polyamide,each of which is directly adhered to one of the two major surfaces ofthe core layer 12; two outer layers 18 and 20, and two adhesive layers14 and 16 each disposed between a respective intermediate layer and arespective outer layer, wherein at least one of the outer layers 18 and20 comprises a blend of a homogeneous or heterogeneous ethylene/alphaolefin copolymer having a density of greater than 0.915 g/cm³ and lessthan 0.925 g/cm³, a homogeneous or heterogeneous ethylene/alpha olefincopolymer having a density greater than or equal to 0.925 g/cm³, and ahomogeneous or heterogeneous ethylene/alpha olefin copolymer having adensity less than or equal to 0.915 g/cm³.

The thickness of the film can vary depending on the end use thereof. Itis preferably between 12 and 80 μm, and more preferably between 14 and60 μm thick. For use as a packaging film, the film thickness ispreferably between 12 and 35 μm, and more preferably between 14 and 26μm, for use in the manufacture of bags the thickness is preferablybetween 35 and 65 μm, and more preferably between 40 and 60 μm.

In the outer layers, the homogeneous or heterogeneous ethylene/alphaolefin copolymer having a density of greater than 0.915 g/cm³ and lessthan 0.925 g/cm³ (component A) is present in the highest percent (byweight) in the blend, and comprises between 35 and 80%, preferablybetween 40 and 70%, more preferably between 45 and 65% by weight of theouter layers. The homogeneous or heterogeneous ethylene/alpha olefincopolymer having a density greater than or equal to 0.925 g/cm³(component B), and the homogeneous or heterogeneous ethylene/alphaolefin copolymer having a density less than or equal to 0.915 g/cm³(component C), each comprise between 10 and 35%, preferably between 15and 30%, and more preferably between 20 and 25% by weight of the outerlayers.

When amounts lower than 10% are employed for component B, the film thatis thus obtained has less desirable mechanical properties. It was foundthat the modulus of films wherein component B is absent in the outerlayers is significantly lower than that of the same films containing atleast 10% of said component in the outer layers.

The use of component C in amounts lower than 10% significantly decreasesthe shrink and heat sealability properties of the film.

Optimal ratios between the various components in the blend are 50:25:25,40:30:30, 60:20:20, 60:25:15, and 50:30:20 (component A: component B:component C).

The thickness of the outer layers is not critical and depends in part onthe overall film thickness and on the number of different layerstherein. When used as a film for packaging, a suitable thickness foreach outer layer is between 3 and 12 μm, while when used for themanufacture of bags for packaging, a suitable thickness is between 8 and20 μm.

At least one of the outer layers preferably contains additives, such asthose conventional additives that are used in small amounts to improveresin processability or the properties of the end film. Examples of saidadditives are antioxidant agents, slip and anti-block agents, UVabsorbers, antimicrobial agents, pigments, anti-fog agents orcompositions, cross-linking agents or cross-link inhibitors, oxygenscavenging agents or compositions, etc.

The density of component A is preferably between 0.918 g/cm³ and 0.922g/cm³. The density of component B is preferably equal to or higher than0.928 g/cm³, and preferably between 0.928 and 0.938 g/cm³. The densityof component C is preferably equal to or lower than 0.912 g/cm³ and evenmore preferably lower than 0.905 g/cm³. Preferably, component C has adensity between 0.895 g/cm³ and 0.912 g/cm³, and preferably between0.898 g/cm³ and 0.905 g/cm³.

The melt flow index (MFI) of the copolymers of components A, B, and C ispreferably between 0.2 g/10 minutes and 10 g/10 minutes, more preferablybetween 0.5 g/10 minutes and 5.0 g/10 minutes, and most preferablybetween 0.8 g/10 minutes and 3.0 g/10 minutes. The use of copolymerswith low MFI increases the mechanical resistance of the film, butnegatively affects resin processability.

Components A and B are preferably a heterogeneous ethylene/alpha olefincopolymer

Component C is preferably a homogeneous ethylene/alpha olefin copolymer.Use of either a homogeneous or a heterogeneous copolymer brings about awidening of the sealability window; however, the width of thesealability window is higher for a homogeneous copolymer.

The sealability window represents in practice the temperature rangewithin which it is possible to seal the film obtaining a substantiallyconstant seal strength above a fixed acceptable lower limit. Since it isoften difficult and sometimes impossible to maintain the sealing bars atthe same temperature, it is in practice necessary to have a sealingwindow as wide as possible to guarantee that almost all the obtainedseals, in spite of the possible and frequent temperature fluctuations ofthe sealing bars, have an acceptable seal strength.

Furthermore, frequently the thermoplastic film plies to be sealedtogether do not lie absolutely flat on one another but some wrinklingoccurs in one or both of the film plies. To guarantee a seal of asuitable strength in all packages it is therefore necessary to increasethe temperature of the sealing bars and/or increase the sealing timewith respect to the theoretical value. A narrow sealing window willtherefore create problems because the temperature reached by the film inthose areas where there are not wrinkles can cause the thermoplasticmaterial to melt, the film to be cut by the pressure of the sealingbars, and the material to be deteriorated with a consequent significantloss in productivity.

It has now been found that by replacing in a prior art film, such asthat described in EP-B-217,596, the ethylene/vinyl acetate copolymer inthe ternary blend of the outer layers with component C, it is possibleto enlarge the sealing window by at least 10° C. By using as component Ca homogeneous ethylene/alpha olefin copolymer, said enlargement of thesealing window reaches 20° C.

In a preferred embodiment of the present invention, the film is anoriented and heat shrinkable film.

Heat shrinkable embodiments of the film have a free shrink, measured at120° C., of at least 20%, preferably at least 30%, and more preferablyat least 40% in at least one direction. Most preferably, heat shrinkableembodiments of the film have a free shrink, measured at 120° C., of atleast 20%, preferably at least 30%, and more preferably at least 40% inboth directions.

The core layer comprises EVOH. Commercial examples are EVAL™ EC F151A orEVAL™ EC F101A, marked by Marubeni.

A single EVOH or a blend of two or more EVOH resins can be employed.Also a blend of one or more EVOH resins with one or more polyamides canbe employed. In this case, suitable polyamides are those commonlyindicated as nylon 6, nylon 66, nylon 6/66, nylon 12, nylon 6,12, andthe like. A preferred polyamide is nylon 6/12, a copolymer ofcaprolactam with laurolactam, such as GRILON™ CF 6S or GRILON™ W8361manufactured and marked by EMS.

In order to improve processability, particularly when a heat-shrinkablefilm is desired, a blend of EVOH with a varying amount of one or morepolyamides is preferably employed. Generally, if a high oxygen barrieris needed, the amount of polyamide blended with EVOH will not be higherthan 30% by weight of the blend. It is however possible to increase thisamount when a limited oxygen barrier is desired. As an example for thepackaging of respiring foods, such as vegetables and cheese, where ingeneral a fair permeability to CO₂ is desired and it is not necessary tohave a high oxygen barrier, it is possible to use blends of EVOH withpolyamides wherein the polyamide(s) are employed in an amount of up to85% by weight on the overall weight of the core layer blend.

The thickness of the barrier layer can vary, depending in part on theoverall thickness of the film and on its use, between 2 and 10 μm. Apreferred thickness is between 2.5 and 5 μm.

The adhesive layers comprise a modified polyolefin as indicated above.Examples of said conventional modified polyolefins are BYNEL™ CXA 4104or BYNEL™ CXA 4105 marked by DuPont, PLEXAR™ 169 marked by Quantum, orsome ADMER™ resins by Mitsui.

It is however been found that when modified polyolefins based onhomogeneous ethylene/alpha olefin copolymers with a density of between0.880 g/cm³ and 0.908 g/cm³ are employed in the adhesive layers, thebond between the layers is remarkably high and there are minimal or nodelamination problems.

Furthermore it was found unexpectedly that by using said materials, andpreferably by using modified polyolefins based on homogeneousethylene/alpha olefin copolymers with a density of between 0.900 g/cm³and 0.908 g/cm³, in at least one of the adhesive layers, a remarkableincrease of the seal strength is obtained and as a consequence thereofan increase of the hot and cold seal resistance.

Examples of said adhesives are ADMER™ AT 1093E (density=0.903 g/cm³ andMFI=1.3 g/10 minutes) and ADMER™ AT 1094E (density=0.906 g/cm³ andMFI=1.5 g/10 minutes) manufactured by Mitsui.

Also the thickness of the adhesive layers may vary depending on theoverall film thickness and on the type of resin employed. In general,adhesive layers having a thickness of between 2 and 8 μm, and preferablybetween 2.5 and 6 μm, are employed.

It was found that the unexpected improvements in the seal strength areobtained in all films comprising an ethylene/alpha olefin copolymer inthe sealing layer.

In a preferred aspect both adhesive layers comprise a modifiedhomogeneous ethylene/alpha olefin copolymer with a density of between0.900 g/cm³ and 0.908 g/cm³.

The films according to the present invention are typically obtained bycoextrusion of the resins and/or blends of resins of the various layersthrough a round or flat extrusion die, quickly followed by quenching atroom temperature. Alternatively, the film according to the presentinvention may be prepared by extrusion coating, wherein one or morelayers are coated, still by extrusion, on top of a first extruded orco-extruded tube or sheet.

If a heat-shrinkable film is desired, the thus obtained thick tube orsheet is heated to the orientation temperature, generally comprisedbetween about 110° C. and about 125° C., by passing it through a hot airtunnel or an IR oven and stretched mono- or bi-axially. When a roundextrusion die is employed, stretching is generally carried out by thetrapped bubble technique. In this technique the inner pressure of a gassuch as air is used to expand the diameter of the thick tubing obtainedfrom the extrusion to give a larger bubble transversely stretched, andthe differential speed of the nip rolls that hold the bubble is used toget the longitudinal stretching. Generally stretching is in a ratio ofat least 3 in each direction. Alternatively, when a flat die is used inthe extrusion, if a heat-shrinkable film is desired, orientation iscarried out by means of a tenter frame. Longitudinal stretching isgenerally obtained by passing the film on at least two couples ofconveying rolls wherein the second set rotates at a speed higher thanthat of the first set. The transverse orientation is on the other handobtained by blocking the film side edges by means of a series of clipsthat travel onto two continuous chains that gradually diverge with theadvancing of the film. Alternatively to said sequential stretching,either longitudinal first and then transversal or transversal first andthen longitudinal, stretching may also be simultaneous in bothdirections. In case of stretching by tenter-frame the stretching ratiosare generally higher than with the trapped bubble method.

Film of the present invention is preferably cross-linked. Cross-linkingmay be achieved either by irradiation or chemically. Radiation involvessubmitting the film to a suitable radiation dosage of high energyelectrons, preferably between 10 and 120 kGrays, and preferably from 20and 90 kGrays.

If a heat-shrinkable film is desired, irradiation is preferably but notnecessarily performed before orientation. If only some of the layers ofthe films need to be irradiated, the extrusion coating technique can beused and the irradiation step carried out on the primary tube or sheet,or the broad beam irradiation system can be used.

When the whole film is cross-linked by electron-beam irradiation, it maybe advantageous to make use of cross-linking controlling agents whichcan be added to the different layers in different amounts to control thedegree of cross-linking in each layer. Suitable cross-linkingcontrolling agents are for instance those described in EP-A-333,294.

Alternatively, chemical cross-linking of the resins can be achieved bythe addition of suitable cross-linking agents, e.g. peroxides, to theresins to be cross-linked. It is also possible to combine chemicalcross-linking and irradiation, as an example when the cross-linkingagents added to the resins need some irradiation to trigger thecross-linking reaction.

The films according to the present invention may optionally be subjectedto other types of energetic radiation treatments which may havedifferent aims. As an example the film may be subjected to a coronadischarge treatment to improve the print receptivity characteristics ofthe film surface.

In case of oriented heat-shrinkable films, it may sometimes be desirableto selectively reduce the shrink force of the thus obtained film, atleast in the transverse direction, without appreciably reducing the %free shrink. This can be useful for instance when the film is used as atray wrapping or a tray lidding. It has in fact been found that withmost of the commercial trays it is advisable to use films having ashrink force in the transverse direction lower than 0.5 N/cm (0.05kg/cm) to avoid tray distortion. In such a case the desired reduction inshrink force may be achieved by subjecting the film obtained by theabove general method to a heat treatment under strictly controlledconditions. In particular such a heat treatment involves heating thefilm to a temperature of from 65 to 95° C. for a time of from 0 to 7.5seconds and then cooling it down to a temperature below roomtemperature, preferably below 20° C., in less than 5 seconds. When across-linked film is desired, such a heat-treatment may be carried out,after orientation, either before or after cross-linking.

The Examples that follow are aimed at better illustrating somerepresentative embodiments of the present invention.

Density is measured by ASTM D 792.

The indicated melting points, if not other wise indicated, aredetermined by DSC analysis following ASTM D 3418 (2^(nd) heating—10°C./min).

Melt Flow Index is measured according to ASTM D-1238, Condition E, at190° C. and is reported as grams per 10 minutes.

In order to evaluate the films according to the present invention thefollowing tests have been used:

% unrestrained free shrink: the % free shrink, i.e., the irreversibleand rapid reduction, as a percent, of the original dimensions of asample subjected to a given temperature under conditions where nilrestraint to inhibit shrinkage is present, was measured according toASTM method D 2732, by immersing for 5 seconds specimens of the films(100-mm by 100-mm) into a bath of hot oil at 120° C. The % free shrinkwas measured in both the longitudinal (machine) and transversedirections. The percent free shrink is defined, for each direction, as

Unrestrained linear shrinkage, %=[(L_(o)−L_(f))/L_(o)]×100

wherein L_(o) is the initial length of side and L_(f) is the length ofside after shrinking.

Shrink tension: the shrink force, which is the force released by thematerials during the heating/shrinking process, when referred to thefilm thickness unit is indicated as shrink tension. There is no standardmethod to evaluate it. It has therefore been measured by the followinginternal method: specimens of the films (2.54 cm×14.0 cm) are cut in thelongitudinal and transverse direction and clamped between two jaws, oneof which is connected to a load cell. The two jaws keep the specimen inthe centre of a channel into which an impeller blows heated air andthree thermocouples measure the temperature. The signal supplied by thethermocouples is amplified and sent to an output connected to the “X”axis of a X/Y recorder. The signal supplied by the load cell isamplified and sent to an output connected to the “Y” axis of the X/Yrecorder. The impeller starts blowing hot air and the force released bythe sample is recorded in grams. The temperature is increased at a rateof 2° C./second. As the temperature increases the pen draws on the X/Yrecorder the measured profile of the shrink force versus the temperaturethus producing a curve of the shrink force (expressed in N) versustemperature (° C.). By dividing the values thus recorded by the specimenwidth (expressed in cm), the shrink force (in N/cm) is obtained. Byfurther diving it by the thickness (in cm) of the sample, it is obtainedthe shrink tension, in N/cm², at the temperature considered.

Haze: haze is defined as the percentage of transmitted light which isscattered forward while passing through the sample and is measured byASTM D 1003 (Method A).

Gloss: the film gloss, i.e. the surface reflectance of a specimen ismeasured according to ASTM D 2457-90 at a 60° angle.

Tensile strength: a measure of the force required, under constantelongation, to bread a specimen of the film, was evaluated by ASTM D882.

Elongation: a measure of the percent extension required to break aspecimen of the film, was evaluated by ASTM D 882.

Tensile Modulus: it has also been evaluated by ASTM D 882—Method A(these last three tests relate to the mechanical properties of the film)

Bond: the load necessary to separate two layers of a structure in apartially delaminated sample, 25 mm wide and 100 mm long, is measuredand given as an indication of the interlayer adhesion. In the specificcase the bond between the core layer and the tie layer was measured.

Broadening of the sealing window: the broadening of the sealing windowwas evaluated using a Omori S5150J Horizontal Form-Fill-Seal machineequipped with a heat sealing bar. The temperature of the sealing bar wasvaried, starting from an average value of 140° C., by decreasing itstepwise by 10° C. at the time. The strength of the seal was evaluatedon batches of 50 packages per each sealing temperature and it has thusbeen determined the lowest sealing temperature that still provides foran effective seal. Then the temperature of the sealing bar was increasedstepwise by 10° C. at the time starting from the average value of 140°C., and the highest sealing temperature, i.e. the highest temperature atwhich the seal does not cut the film, was determined on batches of 50packages per each sealing temperature.

Leaker rate: the improved heat-sealing performance of the filmsaccording to the present invention was evaluated by means of a simpletechnique of leak detection (Dopack system test) based on ASTM D3078-84. In particular this test method evaluates the incidence of“leakers”, i.e. seal defects such as pin-holes which develop at or nearthe seal through which gases escape from or enter into the package. Foreach film, one hundred samples are randomly taken from a production of600 packs obtained on the same packaging machine under the samepackaging conditions. Groups of four packs are then tested by immersingthem in a plastic cylinder filled with water, closing the container,drawing the vacuum and creating a difference in pressure of 3×10⁴ Pa(0.3 bar). In the presence of pinholes, air which was trapped within thepackage will escape giving raise to small bubbles that can be easilydetected and localised. The number of pinholes or “leakers” that is thendetermined is called “leaker rate”. These characeristics have beenevaluated on a Ilapak Delta 2000SB HFFS machine with impulse sealingusing Teflon™ coated sealing wires, a sealing temperature of 170° C., aline speed of 55 packages per minute (corresponding to 18 m/min) andsealing pressure of 2.6×10⁵ Pa (2.6 bar—Condition A) or 3.0×10⁵ Pa (3.0bar—Condition B). These sealing conditions are more drastic than thestandard sealing conditions and allow to better discriminate the sealingbehaviour of the tested structures.

Hot Tack: the hot seal strength was evaluated by a laboratory methodthat simulates what happens on a packaging plant. It is measured bymeans of a dynamometer equipped with hot bars (Hot Tack Tester by TopWave) set as on an industrial packaging machine (in the present case:2.6×10⁵ Pa (2.6 bar ) sealing pressure, 100 ms impulse time, and 250 mscooling time) wherein the sealing temperature is varied. Then thestrength of the seal, in N/mm², is evaluated on 25 mm wide samples andthe sealing temperature range within which the seal strength is above agiven threshold is determined.

EXAMPLE 1

(i) A symmetrical five layer structure was extruded, irradiated at about70 kGrays and biaxially oriented out of hot air at about 116° C.

The resultant 25 μm thick film had a layer ratio of about 3/1/1/1/3 andthe following general structure:

A1+B1+C1/D/E/D/A1+B1+C1

wherein

A1 is a heterogeneous ethylene/alpha olefin copolymer with d=0.920 g/cm³and MFI=1.0 g/10 minutes (Dowlex™ 2045E by Dow)

B1 is a heterogeneous ethylene/alpha olefin copolymer with d=0.935 g/cm³and MFI=2.6 g/10 minutes (Dowlex™ SC2102 by Dow)

C1 is a heterogeneous ethylene/alpha olefin copolymer with d=0.902 g/cm³and MFI=3.0 g/10 minutes (Teamex™ 1000F by DSM)

The A1+B1+C1 blend has 46.6% of A1, 25% of B1, 25% of C1, 3% of ananti-fog composition, and about 0.4% of silica;

D is a homogeneous ethylene/alpha olefin copolymer (Tafmer™ like) withd=0.906 g/cm³ and MFI=1.5 g/10 minutes, modified with maleic anhydride(m.p. 120° C.) (ADMER™ AT1094E by Mitsui), and

E is a blend of 90% of an ethylene-vinyl alcohol copolymer (EVAL™ ECF151A from Marubeni) and 10% of a nylon 6,12 (GRILON™ CF 6S from EMS).

(ii) The obtained film was subjected to a heat treatment that wascarried out on a processing unit consisting of the sequence of 6stainless steel Gross Equatherm heated rollers and two cooled rollers,16-cm in diameter and 203-cm in length, disposed in such a way that thecontact time of the film web with each roller was 0.26 seconds and thetotal heating time 1.56 seconds.

The temperature (° C.) in the three heating zones, each comprising tworollers, was 68.5-68.5-65.3° C. respectively while that in the coolingzone was 20° C. This heat treatment modified the shrink properties ofthe film and in particular it substantially reduced the maximum shrinkforce in the transverse direction of the film leaving almost unalteredthe % free shrink. This treatment did not modify the film sealingproperties.

EXAMPLE 2

(i) A symmetrical five layer structure was extruded, irradiated at about80 kGrays and biaxially oriented out of hot air at about 116° C.

The resultant 25 μm thick film had a layer ratio of about 3/1/1/1/3 andthe following general structure:

A1+B1+C2/D/E/E/A1+B1+C2

wherein

A1 and B1 were as defined in Example 1 and C2 is a homogeneousethylene/alpha olefin copolymer with d=0.902 g/cm³ and MFI=1.0 g/10minutes (m.p. 100° C.) (AFFINITY™ PL1880 by Dow).

The A1+B1+C2 blend contained 46.6% of A1, 25% of B1, 25% of C2, 3% ofanti-fog composition, and about 0.4% of silica;

D and E were as in Example 1.

(ii) The obtained film was subjected to a heat treatment as in part (ii)of Example 1.

EXAMPLE 3

(i) A symmetrical five layer structure was extruded, irradiated at about60 kGrays and biaxially oriented out of hot air at about 116° C.

The resultant 25 μm thick film had a layer ratio of about 3/1/1/1/3 andthe following general structure:

A1+B1+F1/D/E/D/A1+B1+F1

wherein

A1 and B1 were as defined in Examples 1 and 2, and

F1 is an ethylene-vinyl acetate copolymer (about 4% VA).

The A1+B1+F1 blend contained 46.6% of A1, 25% of B1, 25% of F1, 3% of ananti-fog composition, and about 0.4% of silica;

D and E were as in Examples 1 and 2.

(ii) The obtained film was subjected to a heat treatment as described inpart (ii) of Example 1 wherein however the temperature of the heatingzones was 80-80-75° C. respectively.

Table I below reports the characteristics of the films of Examples 1, 2,and 3.

While it can be noted that the mechanical, optical, and shrinkcharacteristics of the films of Examples 1 and 2 were comparable tothose of the film of Example 3 that differs therefrom in the compositionof the skin layers containing an ethylene/vinyl acetate copolymerinstead of component C, the sealability characteristics of the films ofExamples 1 and 2 were remarkably better than those of the film ofExample 3, both in terms of width of the sealing window and in terms ofleakers rate.

TABLE I Film of Ex. no. 1 2 3 Modulus (L-T¹) 539-441 MPa 539-441 MPa520-431 MPa (5500-4500 (5500-4500 (5300-4400 kg/cm²) kg/cm²) kg/cm²)Tensile strength 78-64 MPa 78-64 MPa 78-64 MPa (L-T) (800-650 (800-650(800-650 kg/cm²) kg/cm²) kg/cm²) Elongation 110-140 110-150 110-150(L-T) (%) Free shrink 65-56 64-56 60-56 (L-T) (%) Shrink force0.59-0.34N/cm 0.59-0.34N/cm 0.59-0.39N/cm (L-T) (0.06-0.035)(0.06-0.035) (0.06-0.04) (kg/cm) (kg/cm) (kg/cm) Haze 5 5.1 5.5 Gloss(%) 120 121 122 Bond 1.86N 1.86N 1.86N (190) (190) (190) (g/25 mm) (g/25mm) (g/25 mm) Leakers rate (%) Condition A 7 9 13 Condition B 0 0 10Sealing window 120-160 120-170 130-160 (° C.) Hot tack 15 15 10 (° C.)¹L = longitudinal, T = transversal

EXAMPLE 4

The film of Example 4 was obtained by following the procedure describedin Example 1 but using for the outer layers a blend of two components:A1 and C1 containing 71.6% of A1 and 25% of C1, 3% of an antifogcomposition and about 0.4% of silica.

Table II that follows reports the mechanical characteristics of thefilms of Examples 1, 2, and 4.

The worsening of the mechanical properties and particularly of modulusand elongation of the film of Example 4 with respect to the films ofExamples 1 and 2, that differ therefrom only for the presence ofcomponent B1 in the outer layers, is apparent.

TABLE II Film of Ex. no. 1 2 4 Modulus (L-T) 539-441 MPa 539-441 MPa441-382 MPa (5500-4500 (5500-4500 (4500-3900 kg/cm²) kg/cm²) kg/cm²)Tensile strength 78-64 MPa 78-64 MPa 78-64 MPa (L-T) (800-650) (800-650)(800-650) (kg/cm²) (kg/cm²) (kg/cm²) Elongation 110-140 110-150 120-180(L-T) (%)

EXAMPLE 5

The film of Example 5 was obtained by following substantially the sameprocedure described in Example 1 i)but replacing C1 with C3, ahomogeneous ethylene/alpha olefin copolymer with d=0.915 g/cm³ andMFI=1.0 g/10 minutes (m.p. 108° C.) (AFFINITY™ FM1570 by Dow).

EXAMPLE 6

The film of Example 6 was obtained by following substantially the sameprocedure described in Example 1 i) but replacing C1 with C4, ahomogeneous ethylene/alpha olefin terpolymer with d=0.900 g/cm³ andMFI=1.2 g/10 minutes (m.p. 94° C.) (EXACT™ 3033 by Exxon).

EXAMPLE 7

The film of Example 7 was obtained by following substantially the sameprocedure described in Example 1 i) but replacing C1 with C5, ahomogeneous ethylene/alpha olefin terpolymer with d=0.902 g/cm³ andMFI=2.0 g/10 minutes (m.p. 96° C.) (EXACT™ 9042 by Exxon).

EXAMPLE 8

The film of Example 8 was obtained by following substantially the sameprocedure described in Example 1 i) but replacing C1 with C6, aheterogeneous ethylene/alpha olefin copolymer with d=0.912 g/cm³ andMFI=3.2 g/10 minutes (ATTANE™ 4202 by Dow).

Table III below compares the sealability properties, in terms of leakerrate (Condition A), of the films of Examples 5, 6, and 7.

TABLE III Film of Example no. 5 6 7 Leakers rate (%) 2 8 1

EXAMPLES 9-12

In the following Examples the influence of the resins used in the tielayers of a film otherwise identical was evaluated. By following theprocedure of Example 1 i) and replacing resin D with the materialsindicated in following Table IV as D1 to D4, the films of Examples 9 to12 were obtained. For each of these films in the same Table, the bondbetween the core layer and the tie layer is reported. By using modifiedpolyolefins based on homogeneous ethylene/alpha olefin copolymers withdensity <0.910 g/cm³ in the tie layers a significant increase in bond isobtained.

TABLE IV Bond Film of N/25 mm Example no. Resin employed in the tielayer g/25 mm 9 D1 = maleic anhydride modified homo- 1.4 geneousethylene/alpha olefin copolymer (140) (d = 0.903 g/cm³ - MFI = 1.3 g/10minutes - ADMER ™ AT1093E by MITSUI) 10 D2 = maleic anhydride modifiedhomo- 11 geneous ethylene/alpha olefin copolymer (110) (d = 0.905g/cm³ - MFI = 1.5 g/10 minutes - ADMER ™ AT1072E by MITSUI) 11 D3 =maleic anhydride modified ethyl- 0.8 ene/alpha olefin copolymer  (80) (d= 0.910 g/cm³ MFI = 2.7 g/10 minutes -ADMER ™ NF520E by MITSUI) 12 D4 =maleic anhydride modified ethyl- 0.9 ene/alpha olefin copolymer (d =0.911 g/cm³  (90) MFI = 1.3 g / 10 minutes -ADMER ™ AT1073 by MITSUI)

COMPARATIVE EXAMPLE 13

The film of the Comparative Example was obtained by followingsubstantially the same procedure as in Example 3 with the onlydifference that D was replaced with D5, a maleic anhydride modifiedheterogeneous ethylene/alpha olefin copolymer with d=0.920 g/cm³ (BYNEL™CXA4104 by DuPont).

Following Table V reports both the bond between the core layer and theadhesive layer and the leaker rates of the film of Comparative Example13 as well as of that of Example 3. It can thus be noticed that in thefilm of Example 3 there is an increase in the adhesion between thelayers and also, unexpectedly, a remarkable improvement of thesealability, in terms of leaker rates.

TABLE V Film of Example 3 Comparative Example 13 1.9N/25 mm 0.8N/25 mmBond (190 g/25 mm) (85 g/25 mm) Leaker rate (%) Condition A 13 60Condition B 10 20

The above comparison shows that using a modified polyolefin based on ahomogeneous ethylene/alpha olefin copolymer with a density of from 0.900g/cm³ to 0.908 g/cm³, particular advantages in terms of sealability areobtained not only when the outer layers comprise a ternary blend as inthe preferred films of the present invention but also when the outerlayers in general contain an ethylene/alpha olefin copolymer.

EXAMPLE 14

The film of Example 14 was obtained by following substantially the sameprocedure as in Example 1 i) but increasing the % of A1 from 46.5 to56.5 and decreasing the % of B1 and C1 from 25 to 20%.

EXAMPLE 15

The film of Example 15 was obtained by following substantially the sameprocedure of Example 5 but changing the amount of A1, B1, and C5 in theouter layer as follows: A1 46.5%, B1 35%, and C5 15%.

EXAMPLE 16

The film of Example 16 was obtained by following substantially theprocedure of the foregoing Example but replacing D with D1 as defined inExample 9.

It is to be understood that variations of the present invention asdisclosed can be made without departing from the scope of the invention,which is not limited to the specific embodiments and examples disclosedherein, but extends to the claims presented below.

What is claimed is:
 1. A thermoplastic multilayer packaging filmcomprising a) an oxygen barrier core layer comprising ethylene-vinylalcohol copolymer; b) a first and second outer layer, c) a firstadhesive layer disposed between the core layer and the first outerlayer, and d) a second adhesive layer disposed between the core layerand the second outer layer, wherein the first outer layer comprises ablend of three components A, B, and C, wherein i) component A is ahomogeneous or heterogeneous ethylene/alpha olefin copolymer with adensity greater than 0.915 g/cm³ and less than 0.925 g/cm³, ii)component B is a homogeneous or heterogeneous ethylene/alpha olefincopolymer with density greater than or equal to 0.925 g/cm³, and iii)component C is a homogeneous or heterogeneous ethylene/alpha olefincopolymer with density less than or equal to 0.915 g/cm³, and whereincomponent A is different from component B and component C.
 2. The filmof claim 1 wherein each of the first and second adhesive layers directlyadheres to the core layer.
 3. The film of claim 2 wherein the filmcomprises five layers wherein the surface of each of the first andsecond adhesive layers that does not directly adhere to the core layer,directly adheres to the first and second outer layers respectively. 4.The film of claim 2 wherein the film comprises one or more additionallayers disposed between the first and second adhesive layers and therespective first and second outer layers.
 5. The film of claim 4 whereinsaid additional layers comprise recycle material from the same film. 6.The film of claim 1 wherein polyamide comprising layers directly adhereto the core layer.
 7. The film of claim 6 wherein each of the first andsecond adhesive layers directly adheres to the surface of a respectivepolyamide comprising layer that does not directly adhere to the corelayer.
 8. The film of claim 1 wherein both the first and second outerlayers comprise a blend of three components A, B, and C.
 9. The film ofclaim 1 wherein component A is present in the highest weight percent inthe blend of the first outer layer.
 10. The film of claim 9 whereincomponent A is present in the first outer layer in an amount of between35 and 80% by weight of the blend.
 11. The film of claim 9 whereincomponent B and component C are each present in the first outer layer inan amount of between 10 and 35% by weight of the blend.
 12. The film ofclaim 1 wherein component A has a density of between 0.918 g/cm³ and0.922 g/cm³.
 13. The film of claim 1 wherein component B has a densityof between 0.928 g/cm³ and 0.938 g/cm³.
 14. The film of claim 1 whereincomponent C has a density of between 0.895 g/cm³ and 0.912 g/cm³. 15.The film of claim 1 wherein component A is a heterogeneousethylene/alpha olefin copolymer.
 16. The film of claim 1 whereincomponent B is a heterogeneous ethylene/alpha olefin copolymer.
 17. Thefilm of claim 1 wherein component C is a homogeneous ethylene/alphaolefin copolymer.
 18. The film of claim 1 wherein the film is heatshrinkable.
 19. The film of claim 18 wherein the film has a percent freeshrink, at 120° C., of at least 20% in at least one direction.
 20. Thefilm of claim 1 wherein the first and second adhesive layers comprisemodified homogeneous ethylene/alpha olefin copolymers with a density ofbetween 0.880 g/cm³ and 0.908 g/cm³.
 21. The film of claim 1 wherein thefilm is cross-linked.
 22. The film of claim 21 wherein the film isirradiatively cross-linked.